<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            position: relative;
            width: 100px;
            height: 200px;
            background-color: pink;
            margin: 100px auto;
        }

        .box .tri {
            position: relative;
            top: 10px;
            left: 100px;
            width: 0;
            height: 0;
            font-size: 0px;
            line-height: 0px;
            border: 8px solid transparent;
            border-left-color: pink;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="tri">
            
        </div>
    </div>
</body>
</html>